Colombian president Juan Manuel Santos has signed a new peace agreement with the country's largest rebel movement, aiming to end half a century of hostilities.

Mr Santos and Rodrigo Londono, leader of the Revolutionary Armed Forces of Colombia (Farc), signed the 310-page accord at Bogota's Colon Theatre - nearly two months after the original deal was rejected in a referendum.

The hastily organised ceremony was a far more modest and sombre event than the one in September where the two men signed an accord in front of an audience of foreign leaders and United Nations Secretary General Ban Ki-Moon.

The new accord introduces some 50 changes intended to assuage critics led by still-powerful former president Alvaro Uribe.

They include a commitment from the insurgents to forfeit assets, some of them amassed through drug trafficking, to help compensate their victims.

But Farc would not go along with the opposition's strongest demands - jail sentences for rebel leaders who committed atrocities and stricter limits on their future participation in politics.

In an act of protest, members of Mr Uribe's political party are considering a boycott of next week's scheduled debate in Congress on ratifying the agreement, accusing the legislature of disobeying the constitution.

They are also threatening to call for street protests to denounce what they say is a "blow against democracy".

"The government preferred to impose itself in a way that divides Colombians instead of a national pact that would bring us together," Mr Uribe's Democratic Centre party said in a statement.

The lack of broad support for the accord will make the already-steep challenge of implementing it even tougher.

Colombians overwhelmingly loathe Farc for crimes such as kidnappings and drug trafficking.

Ensuring that the 8,000-plus fighters do not wind up joining criminal gangs rampant throughout the country, or the much-smaller National Liberation Army, will also test the state's ability to make its presence felt in traditionally neglected rural areas at a time of financial stress triggered by low oil prices.

There is also a risk that peace could trigger more bloodshed, as it did following a previous peace process with Farc in the 1980s.

That fear, although less prevalent than in the darker days of Colombia's half-century conflict, has become more urgent with more than a dozen human rights defenders and land activists in areas dominated by Farc being killed by unknown assailants since the initial signing ceremony in September.

Mr Santos this week held an emergency meeting with his cabinet and UN officials to discuss the murders, taking an opportunity to reinforce his message that peace cannot wait.

So far this year, 70 have been killed, according to Bogota-based We Are Defenders, more than in all of 2015 and 2014.

"We have to take action. There's no time to lose," Mr Santos said in a televised address announcing the ceremony.

Once signed, Mr Santos will introduce the accord to congress, where a solid majority in support of peace is expected to ratify it as early as next week.